Physiological characterisation of antennal mechanosensory descending interneurons in an insect (Gryllus bimaculatus, Gryllus campestris) brain
Michael Gebhardt* and
Hans-Willi Honegger
Institut und Lehrstuhl für Zoologie, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany
Present address: Department of Biology, Vanderbilt University, Box 1812, Station B, Nashville, TN 37235, USA
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Fig. 1. (A–F) Morphology of the descending interneurons that receive antennal mechanosensory input. (A) DBNi1-2 is characterised by its large dendrite (arrow), which ramifies extensively in the ventral antennal mechanosensory part of the deutocerebrum. Another dendrite extends anteriorly into the lateral protocerebrum (arrowhead). The axon (asterisk) projects into the ipsilateral connective. ant., anterior; lat., lateral. (B) Sparse projections of DBNi1-2 in the suboesophageal ganglion (sog). (C) Fan-like projections of DBNi1-2 in a typical intracellular fill (Lucifer Yellow). AL, antennal lobe; N1, flagellar nerve 1. (D) DBNi2-1 ramifies extensively in the deutocerebrum (arrow) dorsal to DBNi1-2. Dense arborizations are present in the medial protocerebrum (arrowhead). (E) The DBNc1-2/c2-2 interneuron descends into the connective contralateral to the soma. The single main dendrite is in the ispilateral deutocerebrum. (F) The contralaterally descending DBNc2-3 interneuron is characterised by a primary neurite devoid of any branchings and a fan-like dendrite in the deutocerebrum. All diagrams are in ventral view.
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Fig. 2. (A–D) Response characteristics of DBNi1-2 evoked by forced deflections of the second antennal joint between the scape and pedicel. (A) Compound excitatory postsynaptic potentials (EPSPs) elicited by imposed movements with ramps of amplitude 23° at an angular velocity of 300°s-1 during both adduction and abduction of the antenna. The positions of the antenna at the extreme positions of the deflection are indicated as insets. (B) Correlation between the increase in spike frequency and movement velocity up to 900°s-1 during abduction and adduction between 0 and 100°. Values are means ± S.E.M. of normalised data for 3–7 stimulus periods in five animals. (C) Directional sensitivity to imposed deflections of the scape–pedicel joint (range 0–80°; four steps of 20° each; 300°s-1 as in A). Curves indicate relative mean spike counts (± S.E.M.) per ramp (N=7 animals). The curves are discontinuous every 20°, corresponding to the pauses between two ramps at a constant angular position. Arrows indicate the direction of movement (the outer curves are for movements towards the back of the insect; abduction); the solid line at 30° indicates the antennal resting position. The radius of circles represents the percentage of the maximum response (outer circle, 100%). The shaded sector indicates the range of naturally occurring movements of the scape–pedicel joint. (D) Electrical stimulation of afferent antennal nerves during intracellular recording from DBNi1-2. The shortest latencies (2ms) resulted from stimulation of N1 in the scape (the arrow indicates spikes). Electrical stimulation of the flagellar N1 resulted only in EPSPs of small amplitude and long latency (7ms). Stimulation of N2B failed to elicit any responses. The downward shift in the intracellular trace after stimulation is due to the microelectrode capacitance and was also seen when the electrode tip was extracellular. sc, scape; pd, pedicel; fl1, first flagellar segment; fl6, sixth flagellar segment. Asterisks indicate transient stimulation artefacts. Each recording consists of five superimposed sweeps.
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Fig. 3. (A,B) Response of DBNi1-2 to visual stimuli. (A) Light-off stimuli elicited bursts of spikes. Upper trace, stimulus monitor; lower trace, five superimposed sweeps of the intracellular recording. (B) Mapping of responsiveness to visual input. Sensitivity to the presentation of stationary black dots subtending a visual angle of 8° changes over the receptive field of DBNi1-2. Columns represent relative means of the spike counts (± S.E.M., standardised for each experiment, N=5). DBNi1-2 responded best to dots in the dorso-anterior corner of the computer screen. Note that the the size of the screen did not allow the full size of the receptive field of this neuron to be probed (see Materials and methods).
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Fig. 4. (A–C) Synaptic activity of DBNi1-2 during motor activity. (A) Intracellular recording of DBNi1-2 (lower trace, hyperpolarised with -2nA) and extracellular recording of antennal motoneurons from N4B (upper trace) as a measure of antennal motor activity. Movements of the antenna were not registered. Large excitatory synaptic potentials (arrows) and action potentials (open arrows) are absent during a spontaneous motor neuron burst (arrowhead) and during respiratory-related bursts (asterisks). (B) During forced antennal movements, excitatory postsynaptic potential (EPSP) frequency was also reduced in the presence of high levels of antennal motor activity. Solid bars represent episodes of low synaptic activity in DBNi1-2 and the dashed bar represents episodes of high activity. (C) Box-and-whisker plot showing the median, the second and third quartiles and the fifth and ninety-fifth percentiles of normalised integrals of 176 spontaneous EPSPs plotted against normalised motor spike count in motor nerve N4B from four crickets (see Materials and methods). Large synaptic potentials did not occur in the presence of high motor spike counts. The integral values were binned into classes with a width of 0.1 relative motor spike count units. The maximum integral values decrease with increasing spike count (Spearman rank correlation coefficient r=-0.85, P<0.001). Numbers indicate numbers of samples in each class.
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Fig. 5. (A–E) Responses of DBNi2-1 (A,B), DBNc1-2/c2-2 (C,D) and DBNc2-3 (E) to forced deflections of the ipsilateral scape–pedicel joint. (A) Responses of one DBNi2-1 consisted of spikes. This DBNi2-1 responded best to adductions between 80 and 67° with an average of 23 spikes per ramp. The asterisk marks a spike triggered by a small extra ramp at the medial turning point of the deflection. (B) Average activity over 11 stimulus periods at 90°s-1 for the same interneuron. (C) Recordings from DBNc1-2/c2-2 yielded compound excitatory postsynaptic potentials (EPSPs) and spikes. (D) DBNc1-2/c2-2 was more responsive to adductions than to abductions. Columns represent normalised mean spike counts per ramp (± S.E.M.) of all ramps during adduction versus abduction in five animals (N=23 stimulus periods at 90°s-1; Wilcoxon test P=0.03). (E) Compound EPSPs and bursts of spikes were triggered in one DBNc2-3 by abductions between 80 and 100° at 300°s-1 (five sweeps superimposed). Other angular positions were less effective in stimulating this example of DBNc2-3.
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Fig. 6. (A,B) Electrical stimulation of afferent antennal nerves during intracellular recordings from DBNc1-2/c2-2 (A) and DBNc2-3 (B). (A) Stimulation of N2B (carrying the axons from the scapal chordotonal organ) elicited spikes in DBNc1-2/c2-2 at latencies of approximately 2.5ms. Increasing the stimulus strength from 4 to 6V resulted in an increase in spike count per stimulus and better synchronisation of the first spike. (B) DBNc2-3 received strongest inputs (spikes, marked by an arrow, and excitatory postsynaptic potentials) upon electrical stimulation of N1 in the distal scape, suggesting input from pedicellar proprioceptors. Stimulation of N1 in the third/fourth flagellar segment and of N2B were not effective.
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© The Company of Biologists Ltd 2001
